Those skilled in the art know that the efficiency of cables used to carry power frequency current can be increased by minimizing the circulating currents that are induced in the shielded sections of those cables. Depending upon the distance over which the cables are strung, the cable shield may be bonded together at one or more points. Typically, short lengths of cable have one bond point (single-point bonding) while longer lengths of cable have their shields separated into several segments which are subsequently cross-bonded together. These and/or other bonding methods may be repeated (multi-point bonding) any number of times along the cable route.
It is common for utilities to provide bonding only to the degree required; however, there is no well established criteria when special bonding should be introduced. For single conductor cable circuits, where load currents are above about 500 amps., it is usually more economical to apply some type of bonding since the reduction of losses will permit the use of a lower ampacity cable. In general, cross-bonding is the most effective technique (relative to other bonding techniques) to use for reducing circulating currents and over voltages on the shields. It is also the most expensive to implement. Single point bonding in its many forms is less effective (relative to cross-bonding) at reducing losses due to circulating currents. It also contributes to higher over voltages and is less costly to implement and maintain. Utilities will often employ the less effective, although adequate, single-point bonding when the cable systems are lightly loaded; more effective and complicated cross-bonding method will be employed when loading of the cable is increased (so as to essentially up-grade the system). However, to accomplish this, the interconnections between shields and grounds must be reconfigured.
Although circulating currents can be reduced by single-point bonding, multi-point bonding or cross-bonding, the induced potentials between the shield and the ground are increased. Even though there are no established standards which stipulate how high these shield-to-ground potentials can be raised, safety dictates that the shield-to-ground potential should be kept low. Generally speaking, induced circulating currents and voltages between the shield and the ground are proportional to the magnitude of the current. Voltage surges on the shields comes from lightning, switching, or at the instant of cable failure during service or DC proof-testing. In particular, when a fault current flows, the potential between the shield and the ground can rise to a value which is capable of damaging the power cable and the associated distribution system. The following components may be exposed to these potentially damaging surges: the dielectric at the shield interruption; cable jackets; insulation applied over the metallic shields at the joints; any component connected to the shields or their interconnecting leads; the insulation on the leads connected to those components; and the associated stand-off insulators.
One way of preventing an uncontrolled build-up of voltage is to use surge arresters or over-voltage limiters at suitable locations throughout the cable distribution system. A logical place to provide these voltage limiting devices is at those locations where the cable shield was broken to single-point, multi-point or cross-bond the cables together. Unfortunately, the process of constructing the required electrical connections to the cable shield is a difficult, labor intensive, expensive operation requiring a relatively high degree of skill. Current practice has been to construct the bonding joints and hand-lay a tape covering to restore the insulation system and to re-establish mechanical integrity. Some manufacturers have made available cast iron boxes into which the shield extensions may be introduced and the single point bonding or cross-bonding completed. High voltage cable connectors exist (e.g., U.S. Pat. No. 3,509,518) but not for single-point bonding, or cross-bonding. Those devices are, for the most part, quite large and function simply as ordinary junction boxes. Those skilled in the art know that once the electrical connections are constructed, the finished work must be verified or tested. Ideally, this verification should be accomplished by disturbing the recently completed electrical connections as little as possible or else the test may be meaningless. Imperfect reconstruction of the insulation only reduces the electric reliability of the joint and increases the likelihood of subsequent failure.
It is clearly desirable that a method and apparatus should be provided for simply and rapidly single-point bonding or cross-bonding together the shielded sections of power cables. Moreover, if the apparatus and method incorporates features which allow testing without breaking integrety of the shielded connections, which provide for over-voltage protection, which allow for up-grading of the cable system while reusing existing components, and which allow use in a plurality of single-point bonding or cross-bonding cable protection schemes, then a significant improvement would be contributed to the electrical utility industry.